The invention relates generally to metal articles, and more particularly to a metal article having a multilayer coating, and to methods of making such metal articles. In one embodiment, the invention relates generally to a plain copper foodware article having stick resistance properties and good heat conductivity like pure copper, and more particularly to a plain copper foodware article having a multilayer, durable, stick resistant, ceramic coating, and to a method of making such foodware articles.
Cookware can be made using a variety of base materials, including cast iron, aluminum, stainless steel and clad stainless steel, and clad copper.
Seasoned cast iron cookware has a tough, abrasion resistant surface. However, cast iron is subject to rusting, and it must be cleaned carefully to avoid damage to the cookware surface. In addition, acid in foods can cause leaching of the iron from the surface, which can lead to health problems in some cases.
Copper cookware has excellent heat transfer properties. However, it is easily scratched because it is much softer than other cookware materials, such as cast iron or stainless steel. Copper also oxidizes readily, which leads to tarnishing. Copper can be polished to remove the tarnish, but it requires substantial effort to maintain the surface finish. Furthermore, copper ions can leach into foods.
Aluminum cookware has excellent heat transfer properties. However, aluminum is also subject to leaching of ions. One way to reduce this problem is to coat the surface of the aluminum. Anodized aluminum is coated with aluminum oxide. The oxide layer makes it much harder than untreated aluminum. (Untreated aluminum has a thin layer of aluminum oxide from reaction with oxygen in the air.) However, food will generally stick to anodized aluminum cookware unless oil is used in cooking. In addition, anodized aluminum cookware is not dishwasher-safe, as it can be discolored or corroded by typical automatic dishwashing products.
Aluminum can also be treated by thermal spraying to prevent leaching. However, this process produces a rough surface, and food will generally stick to the surface unless it is treated.
Stainless steel cookware is widely used. It is known for its strength and durability. Stainless steel is relatively easy to clean, and it holds its shine better than copper. However, food is more likely stick to stainless steel than to seasoned cast iron. Overheating, cooking with salt water, or letting the pan “cook dry” causes discoloration of the surface. In addition, although it is typically quite low, leaching of ions, such as iron, chromium, manganese, and nickel, can also be a concern with stainless steel.
In addition, stainless steel is a poor heat conductor. The thermal conductivity of stainless steel at 225° C. is only 19 W/mK, while, at the same temperature, copper is 398 W/mK and aluminum is 250 W/mK. Thus, copper has the heat transfer about 20 times higher than stainless steel.
The performance of stainless steel cookware is largely determined by how well the pan spreads heat, thereby reducing or eliminating “hot spots”. To disperse the heat of stainless steel cookware, an aluminum or copper disc can be added to the bottom of the pan using brazing or impact bonding (the side wall is stainless steel). Although the heat conductivity is improved, the heat distribution of the entire pan is uneven. Further, if the user cooks the food with prolonged overheating, or the pan boils dry, the disc can be separated from the pan.
Another approach to disperse the heat of stainless steel cookware is to use clad stainless steel as the base material of the cookware. Clad material is made of stainless steel and aluminum or copper sandwiched together. These pans conduct heat evenly up the side walls instead of just on the bottom. The clad stainless steel cookware heats quickly, but also cools rapidly because the copper or aluminum core is a good conductor of heat and cold. For example, as shown in FIG. 1, a clad stainless steel pan 5 made of a 5-ply material with a total thickness of 1.87 mm can have an interior 304 stainless steel layer (0.336 mm) 10, an aluminum layer (0.140 mm) 15, a copper core (0.940 mm) 20, an aluminum layer (0.140 mm) 25, and an exterior 430 stainless steel layer (0.31 mm) 30. One example of a 3-ply material with a total thickness of 2.71 mm includes an interior 304 stainless steel layer (0.394 mm), an aluminum core (1.930 mm), and an exterior 430 stainless steel layer (0.386 mm). The heat conductivity of clad stainless steel is a combination of the poor conductivity of the stainless steel layers, which are about 28-35% of total thickness, and the good conductivity of aluminum, or copper.
3-ply and 5-ply stainless steel cookware have the same disadvantages as normal stainless steel cookware: discoloration and scratching of the cooking surface. The cookware can be easily scorched when cooking at high temperature, and cleaning the scorched interior surface takes a lot of time. In addition, the interior stainless steel surface layer can be removed during scrubbing. In many demanding restaurants, a 5-ply skillet will last only about 7-8 months because the copper-core becomes exposed.
Another approach to disperse more heat is to use clad copper as the base material of cookware. As shown in FIG. 2 for example, a pan 40 made of a clad copper material with a total thickness of 2.2 mm can have an interior 304 stainless steel layer (0.4 mm) 45, an aluminum layer (0.3 mm) 50, and an exterior layer of copper (1.5 mm) 55. In this case, the stainless steel layer is reduced to 18% of the total. The resultant heat conductivity of clad copper cookware is better than that of either 3-ply or 5-ply clad stainless steel cookware. However, the interior stainless steel surface has the same problems as other stainless steel cookware: discoloration/scorching and scratching. In addition, the exterior copper surface can readily oxidize. Maintaining the copper surface finish requires substantial effort, as discussed above.
Surface treatments to prevent sticking are known. One well-known surface treatment for cookware involves the use of perfluorocarbon polymers. Perfluorocarbon coatings provide a non-stick surface, but they are easily scratched. Even though current perfluorocarbon coatings are tougher than their predecessors, they are still fairly easy to scratch. When the surface is scratched or nicked, flakes of the perfluorocarbon coating can get into the food being cooked. This flaking is objectionable to many people, despite the fact that perfluorocarbon flakes are not known to pose a health risk. In addition, although the perfluorocarbon polymers are safe at normal cooking temperatures, they can be damaged at high temperatures and may give off toxic fumes.
Another surface treatment involves the use of ceramic coatings. U.S. Pat. No. 5,447,803 describes the deposition of a layer of titanium and a layer of titanium nitride. The titanium nitride coating has high hardness, and a gold color. The titanium nitride coating can be oxidized or nitrided to stabilize the color, but these oxide or nitride coatings are thin and can still be scratched, resulting in possible discoloration of the pan.
U.S. Pat. No. 6,197,438 describes the use of a thick layer (about 2 to 50 microns) of chromium nitride or aluminum nitride as a primer or topcoat layer to achieve scratch resistance and non-stick properties. A decorative or functional top coat layer such as silicon nitride, alumina, or diamond-like carbon can be added. Ceramic coated foodware based on a plasma-sprayed aluminum alloy substrate is also disclosed.
U.S. Pat. No. 6,360,423 describes the deposition of a zirconium nitride coating on cookware. The surface must be polished to a high surface smoothness before the zirconium nitride layer is deposited in order to obtain a stick resistant coating. Although the zirconium nitride coating does not need to be oxidized or nitrided to stabilize the color, zirconium nitride can be discolored in varying degrees by overheating or by salty-based foods.
U.S. Pat. Nos. 6,906,295, and 7,462,375 describe foodware articles having a multilayer, ceramic coating which is stick resistant, scratch resistant, thermally stable, corrosion resistant, and color stable. The foodware article includes a metal foodware article having an inner food-contacting surface and an outer surface. There is a bonding layer deposited on the food-contacting surface, which is typically a metal layer. A layer of (Ti,Al,Cr)N is deposited adjacent to the bonding layer. The coating can optionally include alternating layers of CrN, and (Ti,Al,Cr)N.
There is a need for cookware which has better heat conductivity than clad stainless steel cookware or clad copper cookware, but which does not have the discoloration/scorching and scratching problems associated with the interior stainless surface of clad stainless and clad copper cookware. In addition, there is a need for cookware which has the same heat conductivity as pure copper, and a need for a scratch resistant, stick resistant, thermally stable ceramic coating which has the appearance of metal, which is suitable for use at high cooking temperature, and which can be used with both acidic and salty-based foods.
There is also a need for a ceramic coating with good adhesion to a soft and porous base material, and with good corrosion resistance.